The present invention relates to an optical communication technique, and more particularly to an optical component for dense wavelength division multiplexing communication and a module.
In order to satisfy a requirement for increasing a communication capacity in optical communication, there is often employed a communication method using a wavelength division multiplexing technique capable of increasing a communication capacity by exactly utilizing an existing optical fiber cable. A division multiplexing method having a small multiplexed optical frequency interval of approximately 100 GHz is particularly referred to as dense wavelength division multiplexing (DWDM).
In order to separate a signal obtained by multiplexing the light having such a very small frequency (wavelength) interval, spectral elements such as a wavelength filter or a prism have conventionally been known and are not suitable for dividing a large number of optical beams having close wavelengths. Therefore, a system using an arrayed wave guide grating (AWG) has often been utilized.
However, since the AWG is sensitive to a change in a temperature and an advanced technique is required for fabricating a complicated optical wave guide grating, the AWG is generally expensive and is not suitable for the purpose of requiring a large number of elements.
On the other hand, a diffraction grating is a spectral element in which a fine concavo-convex structure is formed on a surface such as quartz or a silicon substrate. A diffraction light generated in the same structure interferes with each other and a light having a specific wavelength is emitted in a specific direction. For a technique for forming the concavo-convex structure, a photolithography technique to be used for a semiconductor can be utilized. Therefore, it is possible to form a structure with very high precision. Moreover, a replica can also be formed easily through a transfer technique by setting a formed concavo-convex structure to be a master. For this reason, it is apparent that the diffraction grating is an optical component suitable for mass production.
While a device referred to as a wavelength monitor (or a channel monitor) to be used for grasping the state of each channel in a multiplexed light signal is a basically spectral branching filter, a large number of spectral branching filters are incorporated and used in an apparatus, for example, they are utilized for controlling an optical fiber amplifier. The inventors have noted that the diffraction grating is a spectral element suitable for such a purpose and have disclosed, as a spectral branching module, a structure in which a diffraction grating 3 and a photodetector array 4 shown in FIG. 4 are combined (JP-W-WO99/46629). A light beam 10 having a wavelength multiplexed which is emitted from an optical fiber 1 is changed into a parallel light 11 through a collimator lens 2 and is incident on a diffraction grating 3. The light is divided through the diffraction grating 3 and is emitted at an output angle which is varied depending on a wavelength. An emitted light 12 passes through the collimator lens 2 again, thereby forming a focused beam spot group 13 on a photodetector array 4. Each photodetector 40 in the photodetector array 4 is provided in the position of the focused beam spot group 13 of a light having each wavelength (channel).
Assuming that a reflective diffraction grating has a diffraction order of m, a grating period of d and a used wavelength of xcex, an angle formed by a normal on a surface having the diffraction grating formed thereon and an incident beam (an optical axis 5 of an optical fiber) is represented by xcex8i and an angle formed by an emitted beam is represented by xcex8o, the following equation is satisfied.
sin xcex8i+sin xcex8o =mxcex/d 
When xcex8i is constant and a wavelength is changed by xcex94xcex, a change xcex94x in the position of a light beam reaching a light receiving surface provided apart from the diffraction grating by a distance L is given by the following equation.
xcex94x=(Lm/(dxc2x7cos xcex8o))xc2x7xcex94xcex
FIG. 4 shows the case of xcex8l=xcex8o(littrow arrangement). The above equation can be generally established also in the case of xcex8lxe2x89xa0xcex8O. Accordingly, if a wavelength interval is constant, a plurality of photodetectors are arranged at an interval so that the position and wavelength (each channel) of the photodetector can be caused to have one to one correspondence to each other.
A communication wavelength in DWDM is defined to be a frequency pitch every 100 GHz at a minimum by the standard of the International Telecommunication Union (ITU). An example (C band) is shown in the column of a first row from the left of Table 1. For this reason, even if a frequency f is defined at a constant interval based on the relationship of xcex=c/f (c is a light velocity), wavelength intervals are not equal to each other. In the case in which the diffraction grating is used as a spectral element, a position where the divided light beam reaches a light receiving surface is not provided at an interval (see a second row from the left in Table 1).
Moreover, in the case in which a part of channels in the defined frequencies are used, frequency intervals between all the channels do not need to be constant. In this case, a xe2x80x9cjumpxe2x80x9d is caused in the position where the divided light beam might further reach the light receiving surface in addition to the reasons described above.
However, a conventional ordinary photodetector array has a constant pitch between the photodetectors. When such a photodetector is used for the wavelength division multiplexing signal, the photodetector cannot be caused to have one-to-one correspondence with each channel so that a signal for one channel is received by the adjacent photodetectors o r some photodetector elements do not receive a signal. Therefore, there is a problem in that a corresponding relationship between the photoreceptor and the channel becomes complicated and more photodetectors than channels are required for signal separation between the channels. Moreover, there is also a problem in that the area of a photodetector chip is increased corresponding to the presence of the unused photodetector on a chip.
It is an object of the invention to provide a small-sized photodetector array having an excellent signal separation characteristic which solves the problems described above.
In order to attain the object, there is provided a photodetector array in which three or more photodetectors are arranged rectilinearly and an array pitch between the adjacent photodetectors is not constant through the whole array.
Moreover, there is provided an optical communication monitor module in which a diffraction grating having a grating period of d and a diffraction order of m for an incident light having a wavelength interval xcex94xcexi between i""th and (i+1)""th channels is used, an optical path length between the diffraction grating and the photodetector is represented by L and a mean output angle is represented by xcex8o, a pitch P, between the i""th and (i+1)""th photodetectors in the photodetector array satisfies the following equation:
pi=mxcex94xcexiL/d cos xcex8o. 
More specifically, the size of a photodetector array chip can be substantially reduced by changing an array pitch between the photodetectors according to the position of incidence of a light on the photodetector array.
The present disclosure relates to the subject matter contained in Japanese patent application No. 2000-234941 (filed on Aug. 2, 2000), which is expressly incorporated herein by reference in its entirety.